Evolution of Intrinsic Growth and Energy Acquisition Rates. Ii. Trade-Offs With Vulnerability to Predation in Menidia Menidia

Abstract

Abstract. The Atlantic silverside (Menidia menidia) exhibits countergradient latitudinal variation in somatic growth rate along the East Coast of North America. Larvae and juveniles from high-latitude populations display higher intrinsic rates of energy consumption and growth than genotypes from low-latitude populations. The existence of submaximal growth in some environments suggests that trade-offs must counter the oft-cited theoretical benefits of energy and growth maximization (e.g., ''bigger is better,'' ''faster is better'') in the immature life stages. We hypothesized that energy and growth maximization trades off against investment in defense from predators. We conducted laboratory selection experiments to compare vulnerability to predation of silversides from: (1) fast-growing northern (Nova Scotia, NS) versus slow-growing southern (South Carolina, SC) source populations; (2) phenotypically manipulated fast-growing versus moderately-growing NS fish; and (3) recently fed versus unfed NS and SC fish. Tests involved fish drawn from common-garden environments and were conducted by subjecting mixed-treatment schools of sizematched silversides to natural, common piscine predators. NS silversides suffered significantly higher predation mortality than SC silversides. Parallel results were found in phenotypic manipulation of growth: NS silversides reared on a fast-growth trajectory (ϳ1.0 mm/day) were significantly more vulnerable to predation than those growing at a moderate rate (ϳ0.5 mm/day). Food consumption also affected vulnerability to predators: Silversides with large meals in their stomachs suffered significantly higher predation mortality than unfed silversides. Differences in predation vulnerability were likely due to swimming performance, not attractiveness to predators. Our findings demonstrate that maximization of energy intake and growth rate engenders fitness costs in the form of increased vulnerability to predation.Key words. Countergradient variation, evolutionary physiology, genotype-environment covariance, growth rate, latitude, life-history evolution, locomotory performance, optimal foraging theory. At what rate should immature organisms grow to maximize their fitness? Ecological theory suggests that natural selection favors genotypes that maximize net energy acquisition and growth (Lotka 1922;Ware 1982;Stearns 1992). Because rates of survival and reproductive output generally increase with size in many organisms, individuals that grow faster would reap the benefits of large size earlier in life and experience higher survival and fitness (Sauer and Slade 1987;Sogard 1997). Despite evidence and theoretical arguments to the contrary (Priede 1977;Calow 1982;Sibly et al. 1985;Abrams et al. 1996;Arendt 1997;Conover and Schultz 1997;Gotthard 2000), this faster-is-better paradigm is pervasive: Many life-history studies employ growth rate as a direct scalar of relative fitness among juveniles (e.g., Hatfield and Schluter 1999), and habitat quality is routinely evaluated based on positive associations...

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